The genetic genes of organisms are stored in poly deoxynucleotide chains, and proteins that execute biological functions are coded in the genetic genes. Various proteins exist in organisms and they execute different biological functions to maintain vital activities. Though there are numerous kinds of proteins, they are essentially composed of 20 kinds of naturally-occurring amino acids that exist in the natural world. Proteins differ significantly owing to the composition and sequence of these amino acids. Generally speaking, molecules that contain 50 or more amino acids are referred to as proteins, peptide chains that contain 10 or more amino acids are referred to as polypeptides, and peptide chains that contain less than 10 amino acids are referred to as oligopeptides. The smallest functional small-peptide discovered up to now only contains 2 amino acids. Usually, functional peptide fragments that are composed of 4 or more amino acids are commonly seen.
As the Human Genome Project has been completed and the Human Proteome Project has been developed, more and more functional protein segments will be discovered and applied as medicines in the bio-pharmaceutical field. A functional protein segment usually refers to a natural polypeptide segment that is found as having a specific biological function. Such a functional protein segment usually is a peptide segment composed of two to ten amino acids. The identified and discovered functional protein segments can be prepared via an artificial synthesis approach. Polypeptide medicines that have been developed and applied clinically include “oxytocin”, “thymosin al”, and “thymopentin”, etc. Polypeptide medicines available presently include “octreotide”, which is prepared through artificial modification of natural peptide chains and used to treat hemorrhage of the digestive tract and acromegaly, and “hirudin peptide”, which has an anti-coagulation effect. The functional segments in proteins often can be screened for polypeptide segments that contain tens of amino acids or even as few as two amino acids. These functional segments set a basis for artificial synthesis and application of functional polypeptide segments.
In proteins, polypeptides or oligopeptides, the deletion, addition or substitution of a single amino acid, the blocking of an amino terminal (N terminal) or carboxyl terminal (C terminal) amino acid, or the addition of any chemical group into the sequence or at the free end, etc., might result in changes of the original biological activity of the proteins, polypeptides, or oligopeptides. Designing, screening, and discovering new functional peptide fragments or seeking for efficient peptide fragments is an important link in the development of medicines.